This invention relates to alumina based dielectric ceramic compositions. Specifically, the invention is an alumina based dielectric ceramic composition exhibiting a low dielectric constant, a high Q factor, a low temperature coefficient of frequency, and a high percent fired density. The composition requires a relatively lower peak soak temperature during sintering (as compared to conventional alumina ceramics) and is particularly suited for use in high frequency filter applications.
Dielectric ceramic compositions have found use in the field of electronic communications in such components as filters and resonators. In recent years, the range of frequencies used in electronic communications has expanded so that higher frequencies, i.e., those in the microwave range, are increasingly utilized. A filter may be employed to select a signal within a specific frequency range. The frequency range selected by the filter is referred to as the resonant frequency. Such filters may be based upon a block of dielectric material, often a ceramic material. The resonant frequency of the filter is determined by the dielectric properties of that material and by the dimensions of the block. In general, a dielectric material is required which has a low dielectric loss (indicated by a low dielectric loss factor) in order to minimize energy absorption by the dielectric material that would otherwise reduce resonant signal intensity. The Q is defined as the inverse of the dielectric loss factor. Therefore, a relatively lower loss factor results in a relatively higher Q. In general, a higher dielectric constant allows the design of a filter with reduced dimensions. For resonant frequencies above about 2 GHz however, it becomes more difficult to obtain a functional filter because of the small dimensions necessitated by the shorter wavelengths. Thus, a material with a lower dielectric constant, and lower dielectric loss factor (high Q ), is needed in order to maintain the dimensions of the filter in a range conducive to manufacturing limitations. A percentage-fired density approaching 100% also is conducive to achieving a high Q.
Conventional dielectric ceramic materials made of alumina or modified alumina do not exhibit sufficiently high Q values along with sufficiently low temperature coefficient of frequency for satisfactory use as filters and resonators in the microwave frequency band. Additionally, these conventional materials are limited in that they require sintering at relatively high peak soak temperatures of about 1550xc2x0 C. The peak soak temperature is the maximum (peak) temperature achieved during sintering; it is at this temperature that the material remains (soaks) for a period of time.
Furthermore, under normal operating conditions, a filter is typically subjected to a range of temperatures. As temperature changes, the filter""s dimensions are altered by thermal expansion or-contraction of the filter material. This results in a shift in resonant frequency. Dielectric properties are affected by a change in temperature, also tending to shift the resonant frequency. The change in resonant frequency caused by a change in temperature is termed the temperature dependence of the resonant frequency. The temperature coefficient of frequency (Tf) expresses the frequency shift caused by a change in temperature of 1xc2x0 C. For example, a Tf of +5 means that the resonant frequency shifts upward by five ppm with a temperature increase of 1xc2x0 C. A Tf of xe2x88x925 means that the resonant frequency shifts downward by five ppm with a temperature decrease of 1xc2x0 C. A temperature coefficient approaching zero (0) is preferred to minimize the shift in resonant frequency due to variations in operating temperature.
The present invention provides a dielectric ceramic composition comprising a base material represented by the general formula: (x) Al2O3+(y) TiO2, where x and y are percentages of the total weight of the base material, with x being in the range of about 60 to about 96 and y being in the range of about 4 to about 40. Also provided is a first additive material comprising Nb2O5 and a second additive material comprising BaZrO3.
In another embodiment the present invention provides a sintered dielectric ceramic composition comprising a base material represented by the general formula: (x) Al2O3+(y) TiO2, where x and y are percentages of the total weight of the base material, with x being in the range of about 60 to about 96 and y being in the range of about 4 to about 40. Also provided is a first additive material comprising Nb2O5 and a second additive material comprising BaZrO3.